Worm genome offers clues to evolution of parasitism

The genome of a humble worm that dines on the microbial organisms covering the carcasses of dead beetles may provide clues to the evolution of parasitic worms, including those that infect humans, say scientists at Washington University School of Medicine in St. Louis and the Max-Planck Institute for Developmental Biology in Germany.

In a paper published in the current issue of Nature Genetics, the researchers reported finding some surprises as they have decoded the genome of the worm, a tiny nematode called Pristionchus pacificus.

"We found a larger number of genes than we expected," says Sandra Clifton, Ph.D., research assistant professor of genetics and a co-author of the paper. "These include genes that help the worms live in a hostile environment, the result of living in and being exposed to the byproducts of decaying beetle carcasses, and others that also have been found in plant parasitic nematodes. The genome supports the theory that P. pacificus might be a precursor to parasitic worms."

Scientists estimate there are tens of thousands of nematode species. The worms are typically just one millimeter long and can be found in every ecosystem on Earth. Parasitic nematodes can infect humans as well as animals and plants.

One nematode in particular is well known in scientific circles: Caenorhabditis elegans has long been used as a model organism in research laboratories. Its genome sequence was completed in 1998 by Washington University genome scientists working as part of an international research collaboration.

Unlike C. elegans, which lives in the dirt, P. pacificus makes its home in an unusual ecological niche: it lives together with oriental beetles in the United States and Japan in order to devour the bacteria, fungi and other small roundworms that grow on beetle carcasses after they have died. While the beetles are alive and the nematodes' food source is scarce, the worms live in a "resting" stage in which they don't eat or reproduce.

This suspended state, called dauer diapause, is thought to be the infective state of parasitic nematodes. According to the World Health Organization, parasitic nematodes infect about 2 billion people worldwide and severely sicken some 300 million.

The genome of P. pacificus is substantially larger and more complex than C. elegans. It has nearly 170,000 chemical bases and contains 23,500 protein-coding genes. By comparison, C. elegans and the human parasitic nematode Brugia malayi, whose genome was sequenced in 2007, only have about 20,000 and 12,000 protein-coding genes, respectively. Infection with B. malayi causes lymphatic filariasis, which can lead to elephantiasis, a grotesque enlargement of the arms, legs and genitals.

Interestingly, the P. pacificus genome contains a number of genes for cellulases - enzymes that are required to break down cell walls of plants and microorganisms. These genes are nonexistent in C. elegans, although they have been found in plant parasitic nematodes. "Using genetic tools, we can analyze the development, behavior and ecology of this highly unusual worm to aid in understanding the evolutionary changes that allowed parasitism to occur," says co-author Richard K. Wilson, Ph.D., director of Washington University's Genome Sequencing Center.

The P. pacificus genome was sequenced at Washington University; Ralf Sommer, Ph.D., and colleagues at the Max-Planck Institute supplied the DNA for sequencing and analyzed the sequence data.

The research was funded by the National Human Genome Research Institute and the Max-Planck Society.

The genome sequence of Pristionchus pacificus provides a unique perspective on nematode life-style and the evolution toward parasitism.

Washington University School of Medicine's 2,100 employed and volunteer faculty physicians also are the medical staff of Barnes-Jewish and St. Louis Children's hospitals. The School of Medicine is one of the leading medical research, teaching and patient care institutions in the nation, currently ranked fourth in the nation by U.S. News & World Report. Through its affiliations with Barnes-Jewish and St. Louis Children's hospitals, the School of Medicine is linked to BJC HealthCare.